Fiber-optic angular rate sensors, i.e. gyroscopes, are now being implemented to replace mechanical angular rate sensors used for guidance, control and navigation purposes. Fiber-optic devices turn on instantly, have long shelf life and are virtually maintenance free. As "strap-down" devices they do not require expensive gimballed mounting systems and are not encumbered by low rotational-rate lock-in that causes other optical angular rate sensors to provide inaccurate outputs at low angular rates.
The fiber-optic angular rate sensor exploits George Sagnac's concept of exciting an interferometer from an external optical source and directly measuring the phase shift of two counterpropagating light beams after they traverse a closed path. In the case of a fiber-optic device the path is a glass fiber instead of Sagnac's original free space, and the effective sensitivity of the device can be enhanced by using multiple turns of fiber in a coil arrangement.
Basic fiber-optic angular rate sensors use a beam from a single optical source and divide the beam into two beams. The two beams are directed in opposite directions (counterpropagated) through a multi-turn fiber-optic coil. Rotation of the coil produces Sagnac phase shifts in each beam that are equal in magnitude but opposite in sign. The phase difference between the two optical beams is linearly proportional to the rotational rate of the coil. Most configurations use a phase modulator (or frequency shifter) to increase the sensitivity of the device for small angle rotations and to reduce noise by measuring the phase at the modulation frequency.
The principals of the above described technology are described in an article entitled Fiber Optic Gyroscopes by B. Y. Kim and H. J. Shaw published in the March 1986 issue of "IEEE Spectrum", pages 54-60, the same being incorporated herein by reference.
One of the primary motivations for using fiber-optic angular rate sensors is their lack of mechanical parts and wearout mechanisms and hence their potential as a long-life sensor. This is important where reliability is critical, such as in long space missions. The primary present limitation on the life of these devices is the life of the light source. This is especially true in the case where superluminescent diodes are used as the light source.
Accordingly, it is the object of the present invention to increase the expected life of a fiber-optic angular rate sensor by introducing a redundant light source and detector arrangement into a basic angular rate sensor configuration.